The subject matter described herein relates generally to wind turbines and, more particularly, to a method and system for controlling a wind turbine.
Wind turbines generally include a rotor having multiple rotor blades that are attached to a rotatable hub. The rotor transforms wind energy into a rotational torque that drives one or more shafts. Often, the shafts are rotatably coupled to a gearbox that steps up the inherently low rotational speed of the rotor. The gearbox rotates a high speed shaft that drives a generator to produce electrical power, which is fed into a utility grid or to another destination.
At least some known wind turbines are designed to operate within certain torque or power limits to minimize damage to one or more components of the wind turbines due to a force of wind. When wind strikes a wind turbine blade, the force of wind generates a torque and a thrust force on the blade. The torque causes the blade to rotate and produce electrical power. The thrust force, however, may cause damage to the blade and/or to other components of the wind turbine. At least some known wind turbines do not effectively measure the thrust force induced to blades of a wind turbine by the force of wind. To prevent the thrust force from becoming excessive, at least some known wind turbines calculate thrust force from wind speed, and utilize an open loop control system that includes large margins on operating parameters. These large margins may reduce the amount of torque, and therefore power, that can be generated by the wind turbine. As such, known wind turbines often adjust a pitch angle of one or more rotor blades to reduce an amount of energy captured by the rotor blades when the wind speeds are above rated levels.
At least one known wind turbine uses an accelerometer to measure a displacement of the wind turbine tower and/or nacelle due to a force of wind striking the tower and/or nacelle, and to calculate wind speed. However, known accelerometers may be used to measure high frequency motion, but known accelerometers may be less accurate for low speed and/or low frequency motion. For example, at least some large wind turbines may oscillate at about 0.35 Hz or less, and an accelerometer may not be sufficiently accurate to measure such motion. Moreover, at least one known wind turbine includes a control system that monitors a power output of the generator and increases the pitch angle of the rotor blades if the generator speed and/or the power output is near rated levels. However, such control system may cause the wind turbine to operate inefficiently.